The proposed research uses a cross-sectional design to investigate the relationship between the urban, working class, individual and his/her closest sibling from late adolescence to old age. The study will: (1) measure various dimensions of the quality of sibling relationships, (2) describe the differences in sibling relationships during five stages of the adult life cycle, and (3) analyze the extent to which psychosocial variables are predictive of the quality of the relationship. A primary sample of 150 respondents, equally divided by sex, will be administered an open-ended interview and several structured scales, most of which were constructed specifically for the proposed study during preliminary investigations. Elicited data will include measures of the current quality of sibling relationships, psychological and sociological variables which the preliminary studies indicated were relevant to the hypotheses of the proposed study, and retrospective descriptions of sibling relationships at various stages and critical periods during adulthood. A secondary sample, consisting of the closest siblings of respondents from the primary sample, will be asked to complete structured scales measuring personality and the current quality of the sibling relationship so that interactional facets of the relationship, particularly the extent to which similarity or complementarity in personality serve as predictors of the quality of the relationship, can be assessed. The proposed study focuses on an area of adult development that has been explored in only a limited way, and will make significant methodological, theoretical, and practical contributions to the field. The research will be of special concern to mental health practitioners interested in developing and strengthening support networks for the elderly.